feat(sonos): add experimental local playback targets

[mrzhbr]

Summary

• discover Sonos S1/S2 rooms over local SSDP/UPnP and expose them alongside Spotify Connect devices
• persist room selection as sonos:<UUID> and route local transport, seek, volume, metadata, CLI, and playbar behavior through a dedicated Sonos backend
• resolve grouped-room transport and queue operations through the group coordinator while keeping room volume local
• replace stale Sonos queues for new playback, preserve playlist context offsets, and start explicit URI lists immediately while continuation tracks preload asynchronously with cancellation and serialized queue mutations
• pause Sonos explicitly when native, decoded, CLI-selected, or other Spotify playback takes ownership
• validate LAN endpoints, disable redirects, bound response sizes, and provide credential-free SOAP diagnostics

Hardware validation

Validated on standalone S1 and S2 rooms, including multiple-room selection:

• playlist playback and continuation
• Liked Songs playback and continuation
• next/previous, seek, pause/play, and volume
• queue replacement and queue-add behavior
• Sonos-to-native playback handoff
• restart and persisted-room behavior

Grouped-room coordinator behavior is covered by mock integration tests but could not be hardware-tested. Sonos queue viewing and Sonos shuffle/repeat remain intentionally unsupported.

Verification

• cargo fmt --all --check
• cargo clippy --no-default-features --features telemetry -- -D warnings
• cargo test --no-default-features --features telemetry — 519 passed
• cargo clippy -- -D warnings
• cargo test — 738 passed
• cargo clippy --features all-sources -- -D warnings
• LSP diagnostics: clean

Related to #251.

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